Specifically TVS diodes look electrically like two high power zener diodes cathode to cathode with their annodes exposed.

They act exactly in the same was as an MOV Varistor, except that they are much more precise (a 30V MOV may actually trigger at 68V, whereas a 30V TVS will trigger at up to 33V.) They are intended only for surge supression, usually for UL, CSA or IEC compliance testing.

They typically are placed across ANY field wiring circuit and from each field wiring terminal to earth ground.

They do have capacitance which needs to be examined very carefully if you intend to use them across high-speed data lines, including RS-485 and the like.